"stiffened panel" = a panel with stiffeners ...
there are many different reasons to stiffen a panel ... to help the panel carry axial loads (along the stiffeners) or bending loads; or to increase the shear allowable (using the stiffeners to break-up the panel, so called panel breakers).
I'd start with doing my own research (google "stiffened panels" or the like, read aircraft design textbooks (Michael Niu), and come back with a more specific question.
another day in paradise, or is paradise one day closer ?